I see definition of liquid like this:
a substance that flows freely but is of constant volume, having a consistency like that of water or oil. "drink plenty of liquids"
I don’t understand why liquid considered plural, while water not.
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I think you may have looked at the definition from the Oxford Dictionary. It's a bit weird (and confusing) in that it gives the plural of liquids in its first example sentence when the headword is singular.
substance that flows freely but is of constant volume, having a consistency like that of water or oil.
‘drink plenty of liquids’
‘First, you should always drink plenty of liquids (water is the best).’
The word liquids does include lots of types of "wet stuff", and of these water is but one choice. The inference in the first example is that you drink distinct glasses/cups/bottles over a period of time.
And it is possible to have a plural of waters but it's fairly specialised!
The waters of a mineral spring can be used medicinally for bathing in or drinking.
‘You can take the waters at the Pump Room.’
The OED defines the noun liquid in terms of the adjective:
A liquid substance (see A. 1a). In pl[ural] often = liquid food.
What's interesting is that, idiomatically, liquids is shorthand for liquid food (presumably including water). I don't know of a satisfying answer for why this shorthand came about, but the important thing is that plenty of liquids simply means plenty of liquid food. Therefore, liquids isn't actually plural as used, but a mass noun created from a plural.
Even though plenty can modify mass nouns as well as a plural nouns (plenty of air, plenty of noise), this construction does prime the reader to think of liquids as plural. It may help to realize that, although this is a common phrase, it is not itself an idiom, and this sense of liquids is commonly used in other contexts; for instance, a sick person might only be able to consume liquids.